1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer wherein image data is corrected depending on resistance of each heating element of a thermal head. The present invention also relates to a method of measuring resistance of the thermal head and a device therefor.
2. Related Art
A thermosensitive color recording material has been suggested, for example, in Japanese Laid-open Patent Application 61-213169, which has thermosensitive coloring layers for yellow, magenta and cyan which are laminated or formed on a supporting material in this order from the outside. In this type of recording material, the heat sensitivities of the thermosensitive coloring layers (hereinafter referred to as coloring layers) become lower as the distance from the outside surface increases. Furthermore, the coloring layers have properties that each coloring layer is optically fixed by electromagnetic rays of a respective specific wave length range. Therefore, recording of a full-color image on the above-described thermosensitive color recording material is performed in the order from the top or outermost coloring layer to the inner coloring layer, while optically fixing the just recorded coloring layer prior to recording the next coloring layer, so as to avoid undesirable double recording.
The thermal printer has a thermal head having a plurality of heating elements which are connected in parallel to one another and arranged in an array. The thermal head gives a variable amount of heat energy to the color thermosensitive recording layer depending on the sensitivity of the color recording layer to be color developed. Specifically, first a bias heat energy is applied for heating the thermosensitive color recording material up to such a temperature above which a predetermined color begins to be developed in the corresponding color recording layers, the amount of bias heat energy is constant and determined according to the sensitivity of each color recording layer. Next, a variable amount of gradation heat energy necessary for developing the color at desirable density is applied.
To reproduce a fine gradation, it is necessary to accurately control the amount of gradation heat energy. In general, the heating elements are activated or power conducted for about several milliseconds or several ten milliseconds for the bias heating. On the other hand, the conduction time of the heating elements is controlled at an accuracy of several micro seconds or several tens micro seconds.
In spite of such a fine control of heating or conduction time of the heating elements, the consequent image cannot exactly reproduce the desired fine gradation unless all the heating elements of the same thermal head have a completely uniform resistance value. However, in general the heating elements has a variation of about 5% in resistance. For this reason, the printed images tend to have imperfections, such as chromatic unevenness, due to the unevenness of the thermal elements.
To avoid such troubles, a thermal printer has been known, for example, from Japanese Laid-open Patent Application No. 2-248262, wherein resistance values of all the hundreds of heating elements of the thermal head are measured to correct image data based on the results of measurement. In this thermal printer, a capacitor for noise absorption is connected between a pair of power supply terminals through a switch device, and the heating elements are driven by power supplied from the power supply terminals. In a resistance measuring mode, the switch device is turned OFF to inactivate the capacitor. Thereafter, power supply voltage E is applied to one of the heating elements, and a voltage V on that heating element is measured. Then, a resistance value "r" of the heating element is calculated according to an equation r={V/(E-V)}.multidot.R, wherein R is a resistance value of a reference resistor connected between the power supply and the thermal head. This process is executed relating to each heating element, so as to correct image data on the basis of the detected resistance values.
Because the above-described known thermal printer measures the voltage V drop through the heating element, while the noise absorbing capacitor from the power supply is disconnected by turning the switch OFF, the results of measurement tend to be scattered because of the extraneous noise. Furthermore, because the known method requires not only the specific switch but also a device for measuring the voltage V, for example, an analog-digital converter, the construction of the known thermal printer is complicated.